Precise measurement of the successive levels of a signal received by a reception device may be achieved, for example, according to a technique known in itself, by taking from the signal at discrete intervals, several successive samples and by calculating an average of the respective measurements thereof. These groups of samples may be taken, for example, at each period of the signal received. The results are valid providing that the time interval or window during which the samples of a single group are taken has been correctly positioned, on a determined fraction of the signal, half a period for example. Precise positioning of the sampling window with respect to the signal may be achieved with precision if a local clock thoroughly synchronous with the clock signal conveyed with the signal received is available.
This technique for measuring levels from a group of successive samples provides precise results if the transmission conditions are satisfactory. If the signals transmitted are distorted by the propagation thereof on a transmission channel whose passband is inadequate for the rate of data to be transmitted, or if the background noise superimposed on the signal received is relatively high, the level measurements may be altered. In case of a multi-level type transmission applied to the transmission of digitized data where the difference between two successive levels may be relatively small, a level measurement error may lead to confusions in the logic symbols received. The analog signals restored after decoding are therefore distorted.